Not applicable.
The invention relates to methods for calendering tissue paper in general, and more particularly to tissue paper calendering methods involving at least one long belt calendering nip.
Tissue papers contain various sub-grades of silk and tissue papers, such as toilet papers, facial tissues, handkerchief tissues, napkin tissues and wipe-off papers, including e.g. industrial towels, towel papers for use in the kitchen and other towel papers. As is known in the state of the art, tissue papers are conventionally calendered in calendars with a hard nip. The main purpose of calendering is to control the CD (cross direction) thickness. Calendering is performed at very low loads and at a low temperature. In tissue making, calenders are also used in order to bring together the various layers of multi-layer tissue papers before rolling. In connection with the manufacture of some tissue paper grades it has been known to use soft calendering in the calendering of tissue paper, whereby the calendering takes place in a calendering nip, where at least one roll is a so-called soft roll.
U.S. Pat. No. 5,980,691 presents a soft tissue paper with a high density and a method for its manufacture. This publication makes known calendering of a tissue paper web in the calendering nip in such a way that a tissue paper is obtained, the evenness or uniformity of which is less than or equal to 800 microns. In this publication, such a calendering nip is used in the calendering, wherein one roll or both rolls are of steel, surface-coated with rubber, fabric or paper. One roll or both rolls are kept at a temperature, which is optimum for the useful life of the roll in order to prevent overheating of the roll.
As regards the state of the art relating to the invention, reference is also made to Patent Publication WO-9923305, which presents a belt calender, wherein the nip is formed in between the belt and the roll. The belt material used in belt calenders is usually considerably softer than the surface material of soft rolls used in soft calenders.
As is known, the belt material of belt calenders is about ten times softer than the softest surface-coatings of rolls, and it is known in the state of the art that by soft calendering it is possible to achieve significant advantages compared with hard-nip calendering, which is the most commonly used method at present. It is also known that with some paper and board grades the belt calendering concept gives significant advantages in the paper quality compared with soft calendering than soft calendering gives compared with hard-nip calendering. One of the major advantages with board grades is the saving in bulk, since the treatment is very soft and low pressures are used.
The invention aims at bringing about a new method for calendering of tissue paper, with which maximum bulkiness and softness of the tissue paper are achieved.